Hidden orders

When we see quotes in bid/ask, there is something not show. I have got prices better than the bid/ask, that is, between them, with no delay. For example, when the bid/ask is 3.2/3.5, as soon as I transmit a sale order limit 3.25, the order is filled instantly. There seems to be a limit buy order, invisible, for 3.3. The invisible trader has an advantage over the public. Is this true of your experience?